The present invention relates in general to inspection systems and devices for use in assessing the performance of industrial manufacturing processes, and more specifically to a nondestructive inspection or evaluation system for assessing the quality of welds created by laser beam welding.
Laser beam welding is a welding technique for joining multiple pieces of metal through the use of one or more lasers. A laser beam provides a concentrated heat source that allows for the creation of narrow, deep welds at high welding rates. The laser welding process is frequently used in high-volume applications. Similar to electron beam welding, laser beam welding has a high power density (on the order of 1 MW/cm2) resulting in small heat-affected zones as well as high heating and cooling rates. The spot size of the laser used in this process typically varies between 0.2 mm and 13 mm, though only smaller sizes are used for welding. The depth of penetration is proportional to the amount of power supplied, but is also dependent on the location of the focal point of the laser. Weld penetration is maximized when the focal point is slightly below the surface of the workpiece. Continuous or pulsed laser beams may be used depending upon the application. Millisecond-long pulses are typically used to weld thin materials such as razor blades while continuous laser systems are employed for deep welds.
Laser beam welding is a versatile process that is capable of welding carbon steels, HSLA steels, stainless steel, aluminum, and titanium. Resultant weld quality is high, similar to that of electron beam welding, and the speed of welding is proportional to the amount of power supplied, but also depends on the type and thickness of the workpieces. The high power capability of gas lasers make them especially suitable for high volume applications. Laser beam welding is particularly dominant in the automotive industry. However, due to high cooling rates, cracking is a concern when welding high-carbon steels, and welds created by laser beam welding must often be evaluated for weld integrity.
Acoustic methods are commonly used nondestructive testing methods for various inspection applications. Unlike other nondestructive testing methods, acoustic methods provide both surface information and internal information with regard to the welds being evaluated. Moreover, acoustic methods allow for deeper penetration into specimens and higher sensitivity to small discontinuities in a weld joint. Acoustic methods, however, do have certain limitations. The most significant limitations include the requirement of a skillful operator for using the testing device and analyzing acoustic data, as well as the very subjective nature of identifying an inadequate bond or faulty weld joint. Accordingly, the field of ultrasonic nondestructive evaluation (NDE) is in need of a reliable process or technique for identifying poor quality joints in a manner that eliminates the involvement of a skilled operator and the subjective interpretation of test data.